Evidence for frequency dependent diffusion in hydrogel

The electrical response of a hydrogel to an external periodic voltage is investigated by means of the impedance spectroscopy technique. We show that the experimental data, obtained in the linear regime, for the real and imaginary parts of the electric impedance can be interpreted using the Poisson-Nernst-Planck model, taking into account the frequency dependence of viscosity and the non blocking character of the electrodes. We estimate the parameters (ion density, electrode conductivity, and diffusion coefficient) when varying the quantity of salt in the hydrogel. Our experimental data indicate that the bulk density of ions increases with the salt concentration, tending to a saturation value, whereas the electrode conductivity is independent of it. Finally, the parameters defining the dispersion of the diffusion coefficient depend weakly on the salt concentration